Field of the Invention
The present invention relates to a light emitting apparatus comprising a light emitting element which emits fluorescent light or phosphorescent light. In particular, the invention relates to a light emitting apparatus comprising an active element such as insulation gate type transistor or a TFT, and a light emitting element coupled thereto.
Description of the Related Art
A typical display apparatus utilizing liquid crystal uses a back light or a front light for displaying images. A liquid crystal display apparatus is employed as an image displaying unit in various electronics, but suffers a structural problem that it has a narrow angled field of view. On the contrary, a display which uses light emitting elements providing electro-luminescence as a display unit has a wider-angled field of view as well as high level of visual recognition. These advantages make the electro-luminescent display prospective for the next generation.
A light emitting mechanism utilizing the electro-luminescence is considered as a phenomenon in which electrons injected from a cathode and positive holes injected from an anode couple on a layer comprising light emitting material to form excitons and a Light is released when the excitons move back to the ground state. There are two types of electro-luminescence, i.e., fluorescent light and phosphorescent light, each of which is considered as light emitted from the excitons in a singlet state (fluorescent light), and light emitted from the excitons in a triplet state (phosphorescent light), respectively. The luminance from electro-luminescence ranges from thousands cd/m2 to tens of thousands cd/m2, which makes it possible in principle to adopt the electro-luminescence light emitting elements in a variety of applications including a display apparatus.
An example of a combination of a Thin Film Transistor (hereinafter referred to as “TFT”) and a light emitting element is disclosed in the Japanese Patent Laid-Open No. JP-A-8-241047. In the construction disclosed in this JP-A-8-241047, an organic electro-luminescence layer is formed on a TFT comprising polycrystalline silicon, via an insulation film comprising silicon dioxide. A passivation layer having a tapered end on the anode is positioned under the organic electro-luminescence layer. The cathode is made from a material with a work function of 4 eV or less. An example of an applicable material is an alloy of metal such as silver or aluminum, and magnesium.
Known methods for manufacturing the organic electro-luminescence layer include vacuum evaporation, printing, and spin coating. However, it is difficult to form determined patterns on the organic electro-luminescence layer by photolithography technique as used in the semiconductor element manufacturing. In order to arrange the light emitting elements in a matrix to make a display screen, a special construction is necessary in which each pixel is partitioned with insulation material, as disclosed in the above JP-A-8-241047.
In the first place, It was pointed out as a problem that an organic compound used for the light emitting elements, and an alkali metal or an alkali earth metal used for an electrode are degraded by reactions with water and oxygen.
The organic light emitting element deteriorates due to following six factors; (1) change in the chemical characteristics of the organic compound (2) fusion of the organic compounds by heat generated at operating, (3) destruction of insulation due to macro-level defect, (4) deterioration of the interface between the electrodes, or between the electrode and the organic compound layer comprising the light emitting element, (5) deterioration caused by instability in the amorphous structure of organic compounds, and (6) irreversible destruction caused by stress or distortion due to the structure of the elements.
The deterioration by the factor (1) is caused by chemical change incurred by excitation, or gas which are corrosive against the organic compounds or moisture. The deterioration by the factor (2) and (3) is caused by the operation of the organic light emitting element. Heat is inevitably generated when current flowing in the element is converted into Joule heat. When the organic compound has low melting point or glass transition temperature, the electric field concentrates around pinholes or cracks and dielectric breakdown occur. The deterioration by the factors (4) and (5) is inevitable even when the element is stored at ambient temperature. The deterioration by the factor (4) is known as dark spots, which are generated by the oxidation of the cathode or the reaction to moisture. For deterioration by the factor (5), all the organic compounds used in the organic light emitting element are amorphous, so that they will be inevitably crystallized in a long period, and heat. Almost no organic compound can keep its amorphous structure for a long time. For deterioration by the factor (6), a defect such as a crack or a break of the coating due to distortion may develop by the difference in thermal expansion coefficient between components. Furthermore, the crack or the break may lead to a progressive defect such as dark spots.
The advance in sealing techniques has fairly mitigated the problem of dark spots. However in practice, the deterioration is caused by two or more of the above factors, which makes it difficult to take effective preventive measure. In typical sealing method, the organic light emitting element formed over a substrate is sealed with sealant, and drying agent such as barium oxide is applied in the spaces. Unfortunately, conventional preventive measures have failed to suppress the deterioration of the light emitting apparatus to an acceptable level.